The present invention relates to vehicle-wheel alignment measurements, particularly to vehicle-wheel alignment systems which measure the location and orientation of each vehicle wheel in a three dimensional coordinate system utilizing an optical target held on a no-compensation wheel adapter clamp affixed to the vehicle wheel, and more particularly, to an apparatus and method for calibrating the relationship between the optical target and a shaft upon which the optical target is mounted, and for using the calibrated relationship in vehicle wheel alignments.
U.S. Pat. Nos. 5,535,522 and 5,724,743 to Jackson each describe the use of video cameras having one or more defined fields of view to observe optical targets of known configurations mounted to individual vehicle wheels. Through the use of sophisticated image recognition methods, the three dimensional coordinates and orientations of the vehicle wheels and their corresponding axes of rotation are determined. The wheel alignment parameters are determined from these coordinates and orientations.
The relationship between an alignment sensor which is mounted to a vehicle and the axis of rotation of an associated wheel is generally not deterministic, at least not to the level of accuracy required for wheel alignment measurements. The vehicle wheels usually have some amount of runout, which prevents the outer edge of a wheel rim from being representative of a plane normal to the axis of rotation. In addition, the wheel adapter clamps used to mount the alignment sensors to the wheels are usually not manufactured to such tight tolerances so as to guarantee a deterministic relationship between the mounting of the sensor to the clamp and the mounting of the clamp to the wheel.
Accordingly, conventional alignment measurement procedures involve "compensating for runout", which is described, for example, in U.S. Patent No. 3,892,042 to Senften. Certain automobile manufacturers have developed systems to eliminate the need for runout compensation involving specially designed wheels and wheel clamps. Some provide holes in the wheels which allow elongated pins on the clamp to pass through the wheel and press against the surface of the brake disc. Since the disk is presumed to have a surface which is normal to the axis of rotation of the wheel, the mounting of the clamp to the wheel and thus the axis of rotation is controlled. Other methods provide wheels having machined outer surfaces with very tight tolerances and clamps which press against those surfaces, thereby controlling the orientation of the wheel clamp relative to the axis of rotation of the wheel. Clamps for use with such wheels are termed "no-compensation" wheel adapter clamps, as a sensor can be mounted to a wheel in a deterministic manner such that the conventional runout compensation procedure is not necessary.
The disclosures of U.S. Pat. Nos. 5,535,522 and 5,724,743 each discuss determining the relationship between the alignment targets and the corresponding axes of rotation for the wheels each time the alignment targets are mounted to the wheels, but they do not provide for determining the relationships between the targets and the corresponding wheel adapter clamps.
The present invention provides a method for determining the relationship between each alignment target and the mount, for example a stub shaft, by which that target is mounted to a no-compensation wheel adapter clamp. This relationship is stored for subsequent use. Each time a target is mounted to a no-compensation wheel adapter clamp, the relationship for that particular alignment target is applied, and the conventional runout compensation procedure is avoided.
This has several benefits. First, the alignment measurement procedure is faster, as the time normally required to perform the runout compensation procedure is several minutes at best. Second, the vehicle does not have to be jacked up to allow the wheels to rotate, thereby preventing the resulting hysteresis effects from affecting the alignment measurements.